1. Field of the Invention
The present invention relates to thermosensitive recording materials that may allow excellent stampability, prevent sticking, generate less printing residual matters on heads, and represent less image-density decrease due to image-shielding by protective layers.
2. Description of the Related Art
Thermosensitive recording materials have a configuration that a thermosensitive color-developing layer of a thermosensitive recording layer, being possible to color through heating, is disposed on a support such as paper, synthetic paper and resin films. Thermal printers with thermal heads are typically utilized to color through the heating.
Thermosensitive recording methods have various advantages, that is, developing and fixing are unnecessary, devices for the recording are relatively simple, recording period is relatively short and cost for the recording is relatively low, compared to other recording methods, thus are employed in various fields, for example, for POS systems including perishable foods, box lunches and daily dishes; copy systems including books and documents; communication systems such as facsimiles; ticket systems including ticket-vending machines, receipts and acknowledgements.
In recent years, thermosensitive recording materials with thermosensitive recording paper have been developed for ledger sheets along with advancement in information-processing technology using computers and physical distribution technology. Such a thermosensitive recording material is described in Japanese Patent Application Laid-Open (JP-A) No. 55-17529. The thermosensitive recording materials are recently frequently utilized with handy terminal printers in a way that inspectors read meters of electricity, city gas, aqueduct, etc. at individual households and present ledger sheets.
It is important for these handy terminal printers to be printable with lower torque without sticking so as to reduce electric power as low as possible, since the electric sources are typically portable batteries. In these printer systems, recently in particular, high fineness of print information or print dots has been demanded so as to adapt with EAN128 or two-dimensional bar code, consequently, the higher is the dot density of thermal printer heads, the more severe is the requirement for no sticking.
These ledger sheets often contain transfer-payment paper additionally, which requires thermosensitive recording materials to have a proper stampability for this application. Similarly, tickets soled by conductors in trains are also printed using handy terminal printers, and these magnetic recording tickets are also desired for the proper stampability.
JP-A No. 2003-276334 discloses a technology to suppress the sticking in which a molding lubricant based on silicone oil is incorporated in a protective layer; however, employment of silicone oil often suffers from residual matters deposited on heads at printing.
JP-A No. 11-254831 discloses that the sticking may be improved by use of vinyl chloride copolymer as a resin of protective layers. However, mere employment of the vinyl chloride copolymer is likely to be insufficient for low-torque printers represented by handy terminal printers even though the sticking may be sufficiently improved for conventional stationary printers.
Nowadays raw materials with no chlorine or bromine are desired so as to avoid occurrence of halogen gas or dioxin at their incineration, thus the employment of the vinyl chloride copolymer is undesirable from the viewpoint.
JP-A Nos. 01-301368, 10-6647, 2004-268471, etc. disclose technologies for improving the stampability in which inorganic fillers with higher oil-absorption numbers like silica are included in protective layers; however, such inorganic fillers typically raise hardness of the protective layers, which cause a problem of head wear in turn.
On the other hand, JP-A No. 2005-41013 discloses a thermosensitive recording material with excellent stampability, less residual matters deposited on heads, and less head-wear, in which the protective layer includes secondary-agglomerated particles of porous calcium carbonate with an average particle diameter of 2.0 μm or more.
However, the mere employment of the porous calcium carbonate may hardly satisfy the non-sticking as well even though the stampability being satisfied. In addition, JP-A Nos. 2000-177243, 04-341886 and 06-166265 disclose with respect to fillers in protective layers, for example, combination of inorganic fillers and urea/formalin resins, or employment of organic fillers such as porous starch particles; however, these proposals are likely to suffer from much residual matters on heads, decrease of image density due to shielding effect derived from optical scattering by these organic fillers in the protective layers, or the like.
The shielding effect may be suppressed by decreasing the amount of the coating layers, however, which leads to insufficient suppression of the sticking, thus currently the lower-torque printability and the stampability cannot be attained simultaneously.
Incidentally, a technology to incorporate polymethylmethacrylate (PMMA) particles into protective layers is disclosed, for example, in JP-A No. 05-185726, which describes that a thermosensitive recording material may be provided with superior storage stability. However, the exclusive combination of the resin and PMMA particles tends to result in completely insufficient stampability.
In addition, JP-A No. 2005-88457 discloses that the combination of a protective layer, containing 100 parts by mass of polyvinyl alcohol and 100 parts by mass of aluminum hydroxide particles, and 100 parts by mass of silicone rubber particles may improve transporting property under higher humidity conditions, and employment of PMMA particles in place of silicone rubber particles is described as its comparative example.
However, the rate of the resin and the filler disclosed in JP-A No. 2005-88457 also tends to result in completely insufficient stampability and furthermore suffers from much residual matters on heads from the silicone rubber.